Schistura musa, Kottelat, 2017

Kottelat, Maurice, 2017, Three new species of loaches of the genus Schistura from the Nam Ngiep drainage, central Laos (Teleostei: Nemacheilidae), Raffles Bulletin of Zoology 65, pp. 691-706 : 696-700

Schistura musa , new species

( Figs. 10–11 )

Holotype. MHNG 2 7 6 8.0 3 2, 4 2.3 mm SL; Laos: Saysomboune Prov. : Nam Ngiep drainage: Nam Khai [Nam Mang], immediately upstream of confluence with Nam Ngiep , about 22 km downstream of Ban Pou, 19 km upstream of Ban Soppouan; 286 masl, 18°53′49″N 103°28′22″E; M. Kottelat et al., 30 Jan 2014.

Paratypes. CMK 24319, 12 , 25.8–38.5 mm SL (+ 3 ethanol-fixed) ; ZRC 56446, 5 , 26.7–32.5 mm SL; same data as holotype . — CMK 24365, 1 , 41.3 mm SL; Laos: Saysomboune Prov.: Nam Ngiep drainage: Khon District : Nam Pouan [Nam Phouan], upstream of Ban Soppouan , about 300 m upstream of confluence with Nam Ngiep , 244 masl, 18°46′57″N 103°25′58″E; M. Kottelat et al., 31 Jan 2014 . — CMK 24496, 2 , 27.3–29.2 mm SL (+ 1 ethanol-fixed); Laos: Xiangkhouang Prov.: Thathom District: Nam Ngiep north of Ban Naxong , 337 masl, 19°03′24″N 103°22′06″E; M. Kottelat et al., 17 Feb 2014 .

Additional material (non type). CMK 26647, 1 , 43.2 mm SL; Laos: Bolikhamsai Prov.: Nam Ngiep drainage, mixed localities; K. Vatthanatham, March 1999 .

Diagnosis. Schistura musa seems related to S. thavonei with which it shares characters that distinguish them from all other species of the genus in Southeast Asia, like the very elongated body, with a marked hump behind the head (depth at dorsal-fin origin 6.2–8.1 times in its length) and the colour pattern including a pale stripe from the upper extremity of the gill opening to the upper half of the base of the caudal fin (not distinct in all specimens). The black pattern at the base of the caudal fin is made of two asymmetrical slanted marks, one on the base of each lobe, and in front of them a contrasted patch without any pigment extending from dorsal to ventral midlines. Other shared characters useful to distinguish them from other species of Schistura are: male without suborbital flap; 8+7 branched caudal-fin rays; 7½ branched dorsal fin rays; 9–10 pectoral-fin rays; lateral line incomplete, reaching at most to above anal-fin base.

Schistura musa is distinguished from S. thavonei by its colour pattern. In S. musa , the body is usually dull brown; most specimens do not have a clear pattern. Those with a pattern have 13–18 poorly contrasted and very irregularly shaped vertically elongated marks, on lower ½ to 2/3 of flank, from below dorsal fin to caudal-fin base; they become more contrasted posteriorly. In some specimens, a clear barred pattern is present, with 8–9 predorsal bars and 8–10 below and behind dorsal fin. In some specimens, the bars leave a pale stripe between their upper extremity and the darker dorsal midline. In S. thavonei , the colour pattern is made of two broad dark brown stripes (one middorsal, one midlateral) and between them a pale yellowish-brown stripe from the upper extremity of the gill opening to the upper half of the base of the caudal fin; overimposed to the midlateral stripe, a row of 12–24 short black bars, located increasingly lower on the flank from head to tail, posteriormost ones restricted to the lower half of the body or forming blotches along the ventral midline of the caudal peduncle.

Description. See Figs. 10–11 for general appearance and Table 2 for morphometric data of holotype and 6 paratypes. An elongate nemacheilid with body depth increasing up to about midway between occiput and dorsal-fin origin, then decreasing until end of dorsal-fin base, then slightly increasing immediately behind dorsal fin, then tapering to caudal-fin base. Dorsal profile with a concavity between head and body and immediately behind dorsal-fin base. Head slightly depressed; body slightly compressed anteriorly to compressed posteriorly. Interorbital area arched, with a shallow groove above eye. In lateral view, eye flush with dorsal profile of head. Cheeks slightly swollen in holotype. Head and snout pointed in lateral view. Caudal peduncle, slightly tapering posteriorly, depth 1.6–1.8 times in its length. Low dorsal keel on posterior ¼ of post-dorsal area. Low ventral keel on entire length of caudal peduncle. Dorsal keel separated from caudal-fin base by a marked concavity. Largest recorded size 42.3 mm SL.

Dorsal fin with 4 unbranched and 7½ (7*) branched rays; distal margin straight to slightly convex; first or second branched ray longest. Pectoral fin with 1 unbranched and 8 (7*) branched rays (including small last ray, usually unbranched), rounded, reaching about one third of distance to pelvic-fin base. Axillary pectoral lobe present. Pelvic fin with 1 unbranched and 5 (1) or 6 (6*) branched rays (including small last ray, usually unbranched); reaching about ½ to 2/3 of distance to anus; rounded; origin slightly in front of dorsal-fin origin. Axillary pelvic lobe present, free. Anus situated about 1.5–3 eye diameter in front of anal fin. Anal fin with 3 unbranched and 5½ (7*) branched rays; distal margin straight to slightly convex. Caudal fin with 8+7 (6) or 8+6 (1*, lowermost ray damaged, was probably two branched rays); about 3–5 dorsal and 3–4 ventral procurrent rays (cannot be counted with accuracy); forked, lobes rounded, subequal, lower lobe slightly longer than upper one.

On pectoral fin, thick unculiferous pad (sensu Conway et al., 2012) along anterior edge of unbranched ray; on dorsal side a pad on each membrane along posterior edge of unbranched ray and branched rays 1–4; on ventral side, a pad on membranes along anterior edge of branched rays 1–4. On pelvic fin, a pad along anterior edge of unbranched ray; on dorsal side a pad on membranes along posterior edge of branched rays 1–3; on ventral side, a pad on membranes along anterior edge of branched rays 1–3. On anal fin, a pad along anterior edge of last unbranched ray.

Body covered by scales except on belly in front of anus or pelvic-fin base. In largest specimens scales in predorsal area sparsely distributed, missing in smaller specimens. Scales deeply embedded. Lateral line incomplete, often interrupted, reaching between pelvic-fin base and middle of caudal peduncle (posterior extent increasing with size), with 40–91 pores (difficult to count with accuracy because of gaps). Cephalic lateral line system with 5–6 supraorbital, 4 + 9–10 infraorbital, 9 preoperculo-mandibular and 3 supratemporal pores.

Anterior naris pierced in front side of a flap-like tube. Posterior naris adjacent to anterior one. Mouth arched, gape about 2–3 times wider than long ( Fig. 12 ). Lips thin. Upper lip with median notch (small to conspicuous), without furrows, edge not crenulated. Processus dentiformis present. Lower lip with median interruption; median part with 1–3 sulci, lateral part smooth. Tip of lower jaw not exposed. No median notch or concavity at tip of lower jaw. Inner rostral barbel not reaching base of maxillary barbel; outer one reaching vertical of posterior nare. Maxillary barbel reaching at most vertical of anterior half of eye. Intestine almost straight behind stomach ( Fig. 13 ). Air bladder without posterior chamber in abdominal cavity.

Sexual dimorphism. None observed, but in holotype (largest specimen), the first branched ray of the pectoral fin is clearly thicker (especially the branches) than the other rays and than in other specimens. Thickened pectoral-fin rays are observed in males of many nemacheilids and are potentially sexually dimorphic in S. musa too. No specimen with suborbital flap, groove or slit.

Coloration. After 6 weeks in formalin. Head and body background colour pale brown, throat, belly, lower part of caudal peduncle pale yellowish brown; except otherwise stated, markings darker brown. Head without pattern. In most specimens body without clear pattern. A row of 13–18 poorly contrasted and very irregularly shaped vertically elongated marks, on lower ½ to 2/3 of flank, from below dorsal fin to caudal-fin base; becoming more contrasted posteriorly ( Fig. 11 ). In four largest specimens, a clear barred pattern, with 8–9 predorsal bars and 8–10 below and behind dorsal-fin, wider than interspaces ( Fig. 10 ). Postdorsal bars wider than predorsal ones, and appearing as resulting from merging of several of the irregular marks of smaller individuals. Predorsal bars increasingly less distinct anteriorly. Postdorsal bars not reaching dorsal and ventral midlines. Anterior bars merged into colour of back. In some specimens, bars leaving a pale stripe between their upper extremity and darker dorsal midline ( Fig. 11 ). A wide inner axial stripe present on posterior half of body in some specimens.

Black pattern at caudal-fin base: last bar on caudal peduncle separated from caudal-fin base by a narrow yellowish space, leaving a vertical band devoid of any pigment. On upper half of base: a patch of black deep pigments on basal part of upper 4–5 principal rays and on last 2 procurrent rays (on rays only, not on fleshy base of fin). Anterior edge usually more posterior than that of patch on lower half of fin. On lower half: a patch of black deep pigments on basal part of lower 3–4 principal rays. A roundish spot of small black pigments at middle of base of fin, about ¼ of its depth, in two largest specimens merged with or overimposed on last bar of caudal peduncle ( Fig. 10 ). Posterior part of dorsal keel dark brown, leaving a vertical unpigmented patch running from concavity between keel and caudal fin, in front and/ or behind dark spot and to ventral mid-line.

Dorsal fin hyaline, with a small black spot at base of simple rays and first branched ray, continued as a low dark grey band along whole base of fin. Black pigment on each branched ray around branching point and on distal half of last unbranched ray. In holotype, a patch of black pigments on membranes between last unbranched rays and branched ray 3, on one or the other side. Caudal fin hyaline, with pigments along edges of all rays. Anal, pelvic and pectoral fins hyaline, with a few pigments near branching points of branched rays.

Notes on biology. A dissected female (CMK 24319, 37.4 mm SL) had ovaries with white oocytes (estimated less than 50) and a few yellow ova about 0.7 mm diameter. A male (CMK 24319, 29.0 mm SL) had ripe testes; its stomach was filled with numerous small insect larvae. The largest sample was obtained at the mouth of a small tributary, 3–5 m wide, 20–50 cm deep, with moderate to fast current, over a pebble to stone bottom in an uninhabited area ( Fig. 14 ). In the same area, a single specimen was obtained in the Nam Ngiep itself, among stones along the shore. Two specimens were collected in a stretch where the river is wider, shallower and with a weaker current; the bottom was made of pebbles and stones, but was heavily silted with mud and stand, apparently originating from work in the river at construction sites or hydropower dam(s) upriver (Google Earth images dated 23 January 2014 [3 weeks before sampling] show clear-water tributaries entering a very turbid Nam Ngiep).

Distribution. Schistura musa has been observed only in the middle Nam Ngiep ( Fig. 8 ); the sections upstream and downstream could not be sampled. It has not been observed in the reasonably well surveyed Nam Ngum drainage to the west and in the insufficiently sampled Nam Xan to the east.

Etymology. Musa is the genus name of the bananas (Musa spp.). A reference to the shape of the preserved specimens. A noun in apposition.

Conservation status. Schistura musa was observed at three localities in 2014. It was observed in two tributaries of the Nam Ngiep in the area that will be flooded by the Nam Ngiep 1 reservoir; it is expected to be present further up in these tributaries, in stretches with low gradient, as well as in unsurveyed parts of the Nam Ngiep drainage. These streams could not be sampled further upstream because of access difficulties and serious security issues. Because of its apparent preference for stretches with relatively moderate current, S. musa is not expected to reach very far upstream along these tributaries. It was also collected in the Nam Ngiep itself, above the maximum level of the reservoir (320 masl), but there the bottom of the river is covered by sediments apparently originating from construction work at the sites of Nam Ngiep 2 hydropower scheme and road construction. As a result, all known sites are or will be inundated or covered by sediments at least part of the year and the survival of the species at these sites is unlikely. It may still be able to survive in some still unsampled tributaries of the drainage but its presence first has to be demonstrated. However, these are small water bodies with limited flow and they are not expected to be able to support large populations of large adults. The sharp decrease in range, habitat quality and population size probably qualifies S. musa to the Endangered or Critically Endangered categories under IUCN criteria (IUCN, 2001).

Figures

Fig. 10. Schistura musa, MHNG 2768.032, holotype, 42.3 mm SL; Laos: Saysomboune Prov.: Nam Ngiep drainage: Nam Khai; right side, reversed.

Fig.11.Schistura musa, CMK 24319,paratypes; Laos:Saysomboune Prov.: Nam Ngiep drainage: Nam Khai; a, 31.3 mm SL; b, 38.5 mm SL.

Fig. 12. Schistura musa, MHNG 2768.032, holotype, 42.3 mm SL; mouth.

Fig. 13. Schistura musa, CMK 24319, 29.0 mm SL; digestive tract. Scale bar = 1 mm.

Fig. 14. Nam Khai, Nam Ngiep drainage, Saysomboune Province, Laos; type locality of Schistura musa; 30 January 2014.

Fig. 8. Laos: Nam Ngum, Nam Ngiep and Nam Xan drainages, showing distribution of Schistura albisella (circles) and S. musa (triangles). Open symbols: type localities.